1. Field of the Invention
The invention pertains to the field of corrosion protection of structures. More particularly, the invention pertains to methods and apparatus for protection of structures against corrosion using impressed cathodic current.
2. Description of Related Art
Oftentimes metal structures are protected from the effects of environmentally-sourced corrosion through the implementation of Impressed Current Cathodic Protection (“ICCP”) systems. Conventional deployment of these systems is realized with an open loop control topology, where the DC power source (i.e., the “ICCP Rectifier”) output potential is fixed based on a manual survey of the structure to determine appropriate protection levels. FIG. 1 presents a typical prior art deployment scenario for a well casing cathodic protection system 10.
The well casing 12 extends into the ground with a well head 14 extending above ground level 16 from the well casing 12. A cathode junction box 20 is located at or above ground level 16 and is coupled to the well head 14 by a negative connection 22. The cathode junction box 20 is also coupled to a pole-mounted cathodic protection rectifier 30 by a second negative connection 24. The cathodic protection rectifier 30 is also coupled to an anode junction box 40 by a positive connection 42. The anode junction box 40 is also coupled to a deep anode bed 50 by another positive connection 44. The deep anode bed 50 includes backfill 52 that is electrically conductive and a plurality of anodes 54 extending down the center of the deep anode bed 50 through the backfill 52.
The deep anode bed is often of the type sold under the trademark EnvirAnode® with AEL™ anodes and Conducrete® backfill (SAE, Inc., Barrie, Ontario, Canada).
A major limitation of this approach is that it does not account for seasonal variations in surrounding media electrical resistance. This limitation forces the system operator to manually check the system's performance on a regular basis and implement a manual readjustment of the system's output voltage to its optimal value.
Conventional systems also do not prevent certain sections of the structure from being “over-protected” due to local surrounding media electrical resistance variations. This can result in potential structure embrittlement and ultimate compromise.
The conventional practice is to utilize utility line frequency transformers, followed by discrete rectification and filtering components to realize the DC power source required to implement an ICCP system. These components are typically housed in an environmentally-sealed cabinet that protects the circuitry from external environmental effects.
More often than not, these conventional systems do not employ an automatic, closed-loop feedback system to regulate output power, but instead rely on manual setting of output power (voltage and/or current) via taps on the transformer. These conventional systems also present challenges for field maintainability, requiring technicians to troubleshoot and repair circuits to the component level if there is a failure, thus presenting issues with regard to training and safety of personnel, as well as the potential for excessive equipment down time.
Vukcevic (“A Novel ‘Green’ Approach to Powering Marine ICCP Systems”, presented at Corrosion & Prevention 2008 Conference, Wellington, New Zealand, Nov. 16-19, 2008) discloses a DC power distribution system incorporating Current Multiplier DC/DC converters (CM-PDS) in the construction of sea water ICC systems. The disclosed CM-PDS system approach has a modular structure, based on standardized building blocks. A major feature of the disclosed approach is dual power transformation before DC current reaches the anodes, as a high voltage distribution network is introduced between the AC/DC power supplies and Current Multiplier converters feeding the anodes. By distributing high voltage/low currents, the low voltage distribution cable network is eliminated as a major source of power losses associated with ICCP.
The above-mentioned reference is hereby incorporated by reference herein.